Documentation

Normally when an exception is encountered while propellor is
ensuring a property, the property fails, but propellor robustly
continues on to the next property.

This is the only exception that will stop the entire propellor run,
preventing any subsequent properties of the Host from being ensured.
(When propellor is running in a container in a Host, this exception only
stops the propellor run in the container; the outer run in the Host
continues.)

You should only throw this exception when things are so badly messed up
that it's best for propellor to not try to do anything else.